The present invention relates to a device for reducing the structural-acoustic coupling between the diaphragm vibration field and the enclosure acoustic modes in a small speaker. In particular, the present invention relates to a modified acoustic cap.
In systems having small speakers, such as telephone sets, cost is an important issue. Small, inexpensive loudspeakers having a size of 50 to 60 mm are typically used. In order to produce enough sound power given the mass of the diaphragm, both the stiffness of the cone edge and the damping tend to be low. Therefore, the diaphragm has a high mobility.
Due to the dimensions of the telephone sets or small speakers, acoustic resonances can occur in the enclosure in the frequency band of interest, 300-3400 Hz for traditional telephony, and 150-7000 Hz for wide-band telephony. The coupling of the loudspeaker diaphragm with the acoustic modes (resonances) in the enclosure produces unwanted effects on the global sound receive curve in the frequency band of interest. This coupling results in notches that have an amplitude which depends on the loudspeaker diaphragm damping, diaphragm stiffness and on its position relative to the enclosure acoustic modeshapes.
For cost and manufacturing reasons it is typically undesirable to use acoustic damping, such as foam or a similar material, in the enclosure to limit acoustic resonances.
The inventors are unaware of any devices that have been designed that provide an alternative to the use of an enclosure treatment: U.S. Pat. No. 5,150,418 to Honda et al. discloses a cap having a bass-reflex, which attempts to widen the loudspeaker frequency response. U.S. Pat. No. 4,618,025 to Sherman discloses a cap provided in a speaker enclosure that attempts to dampen the diaphragm and lower its first resonance frequency. The prior art does not contemplate controlling the coupling between the loudspeaker diaphragm and acoustic modes in the enclosure in order to modify the acoustic response.
It is therefore an object of an aspect of the present invention to provide a device that can be used to control the coupling between the loudspeaker diaphragm and acoustic modes in the enclosure in order to modify the global sound receive curve in the frequency band of interest.
According to one aspect of the present invention there is provided a housing for an acoustical speaker having a movable diaphragm. The housing comprises an outer casing having an aperture, a cap having a flange located at an outer edge thereof, the flange being coupled to the outer casing so that the cap covers the aperture, and a cavity provided in the cap, the cavity being sized to house the acoustical speaker. The cap de-couples the diaphragm from the acoustic resonances in the outer casing. A gap is provided between the cap and the outer casing which dampens a first resonant frequency of the diaphragm without strong coupling to the acoustic resonances.
Preferably, the flange of the cap comprises at least one protrusion extending from the flange for abutting the outer casing, wherein the gap is provided between the flange and the outer casing delimited by the protrusion.
It is an advantage of an aspect of the present invention that the coupling between the loudspeaker diaphragm and acoustic modes in the enclosure is controlled thus, the acoustic response can be controlled.
It is a further advantage of an aspect of the present invention that the diaphragm resonance peaks, primarily the first one, are dampened, which widens the speaker sound response in the low frequency end.